Processors often use a MESI state coherency mechanism for multiprocessor caching, where every line in the cache is marked either Modified, Exclusive, Shared, or Invalid. A shared line can be converted to Exclusive or Modified (when a store wants to retire to a shared line, for example) only after first sending a snoop to other caches that converts all other possible shared lines in peer caches to Invalid, and then finally allowing the line to be upgraded to Exclusive. The possible state combinations are tracked and implemented in accordance with the MESI state coherency mechanism.
The problem with a conventional system, however, when a store hits a shared cache line, the system has two choices. It can either wait for all snoops to complete before allowing the store to complete, which reduces the parallelism of the machine, or it can evict the shared line, then complete the store to an invalid line. This allows more parallelism, because the processor can continue to execute other code, but does not allow any later loads to see the data contained in the line. It also adds latency to perform the eviction.